给出二十个c++考试常考的函数，给出具体代码例子

好的，以下是二十个C++考试常考的函数以及代码示例：

1. strlen：用于计算字符串长度

#include <iostream>
#include <cstring>
using namespace std;

int main() {
    char str[] = "Hello, world!";
    int len = strlen(str);
    cout << "The length of the string is: " << len << endl;
    return 0;
}

2. strcpy：用于将一个字符串复制到另一个字符串

#include <iostream>
#include <cstring>
using namespace std;

int main() {
    char str1[] = "Hello";
    char str2[10];
    strcpy(str2, str1);
    cout << "The copied string is: " << str2 << endl;
    return 0;
}

3. strcat：用于将一个字符串追加到另一个字符串的末尾

#include <iostream>
#include <cstring>
using namespace std;

int main() {
    char str1[] = "Hello";
    char str2[] = " world!";
    strcat(str1, str2);
    cout << "The concatenated string is: " << str1 << endl;
    return 0;
}

4. toupper：用于将小写字母转换为大写字母

#include <iostream>
#include <cctype>
using namespace std;

int main() {
    char ch = 'a';
    char upper_ch = toupper(ch);
    cout << "The uppercase letter is: " << upper_ch << endl;
    return 0;
}

5. pow：用于计算幂次方

#include <iostream>
#include <cmath>
using namespace std;

int main() {
    double base = 2.0;
    double exponent = 3.0;
    double result = pow(base, exponent);
    cout << "The result is: " << result << endl;
    return 0;
}

6. abs：用于计算绝对值

#include <iostream>
#include <cstdlib>
using namespace std;

int main() {
    int x = -5;
    int abs_x = abs(x);
    cout << "The absolute value of " << x << " is: " << abs_x << endl;
    return 0;
}

7. atoi：用于将字符串转换为整数

#include <iostream>
#include <cstdlib>
using namespace std;

int main() {
    char str[] = "123";
    int num = atoi(str);
    cout << "The converted integer is: " << num << endl;
    return 0;
}

8. atof：用于将字符串转换为浮点数

#include <iostream>
#include <cstdlib>
using namespace std;

int main() {
    char str[] = "3.14";
    double num = atof(str);
    cout << "The converted float is: " << num << endl;
    return 0;
}

9. sqrt：用于计算平方根

#include <iostream>
#include <cmath>
using namespace std;

int main() {
    double x = 4.0;
    double square_root = sqrt(x);
    cout << "The square root of " << x << " is: " << square_root << endl;
    return 0;
}

10. ceil：用于向上取整

#include <iostream>
#include <cmath>
using namespace std;

int main() {
    double x = 3.14;
    double ceil_x = ceil(x);
    cout << "The ceiling of " << x << " is: " << ceil_x << endl;
    return 0;
}

11. floor：用于向下取整

#include <iostream>
#include <cmath>
using namespace std;

int main() {
    double x = 3.14;
    double floor_x = floor(x);
    cout << "The floor of " << x << " is: " << floor_x << endl;
    return 0;
}

12. round：用于四舍五入

#include <iostream>
#include <cmath>
using namespace std;

int main() {
    double x = 3.6;
    double round_x = round(x);
    cout << "The rounded value of " << x << " is: " << round_x << endl;
    return 0;
}

13. srand：用于初始化随机数种子

#include <iostream>
#include <cstdlib>
#include <ctime>
using namespace std;

int main() {
    srand(time(0)); // 初始化随机数种子
    int random_num = rand() % 100; // 生成一个0-99的随机数
    cout << "The random number is: " << random_num << endl;
    return 0;
}

14. getline：用于读取一行字符串

#include <iostream>
#include <string>
using namespace std;

int main() {
    string str;
    cout << "Enter a string: ";
    getline(cin, str);
    cout << "The string you entered is: " << str << endl;
    return 0;
}

15. cin.ignore：用于忽略输入流中的字符

#include <iostream>
#include <cstdlib>
#include <ctime>
using namespace std;

int main() {
    srand(time(0)); // 初始化随机数种子
    int random_num;
    cout << "Guess a number between 1 and 100: ";
    cin >> random_num;
    cin.ignore(100, '\n'); // 忽略输入流中的字符
    int actual_num = rand() % 100 + 1;
    if (random_num == actual_num) {
        cout << "Congratulations! You guessed the number!" << endl;
    } else {
        cout << "Sorry, the actual number was: " << actual_num << endl;
    }
    return 0;
}

16. isalpha：用于判断字符是否为字母

#include <iostream>
#include <cctype>
using namespace std;

int main() {
    char ch = 'a';
    if (isalpha(ch)) {
        cout << ch << " is a letter" << endl;
    } else {
        cout << ch << " is not a letter" << endl;
    }
    return 0;
}

17. isdigit：用于判断字符是否为数字

#include <iostream>
#include <cctype>
using namespace std;

int main() {
    char ch = '5';
    if (isdigit(ch)) {
        cout << ch << " is a digit" << endl;
    } else {
        cout << ch << " is not a digit" << endl;
    }
    return 0;
}

18. to_string：用于将数字转换为字符串

#include <iostream>
#include <string>
using namespace std;

int main() {
    int num = 123;
    string str = to_string(num);
    cout << "The converted string is: " << str << endl;
    return 0;
}

19. stoi：用于将字符串转换为整数

#include <iostream>
#include <string>
using namespace std;

int main() {
    string str = "123";
    int num = stoi(str);
    cout << "The converted integer is: " << num << endl;
    return 0;
}

20. stof：用于将字符串转换为浮点数

#include <iostream>
#include <string>
using namespace std;

int main() {
    string str = "3.14";
    float num = stof(str);
    cout << "The converted float is: " << num << endl;
    return 0;
}